Convert into a composite action

[helen]

Description of the Change

Failures with building the environment due to something going on with Debian (stable-slim), always felt that it was overly expensive for the actual needs of this action which is "just" a Bash script. The GitHub runners generally include all things necessary for this action - SVN, Git, rsync, and zip. Composite run steps are a thing as of about a year ago, and looks like as of the last couple months there is more official support for them being used as actions themselves. See https://docs.github.com/en/actions/creating-actions/creating-a-composite-run-steps-action

Alternate Designs

Switching from Debian to Ubuntu, but this still has issues around building the image, etc. We could cache it, but that is getting into wild overkill territory IMO.

Benefits

Faster runs with fewer dependencies

Possible Drawbacks

Not sure right now, need to test to see how exactly inputs and environment variables work with this method.

Verification Process

Will be testing on my test repo.

Checklist:

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests passed.

Applicable Issues

Fixes #73, fixes #60

Changelog Entry

Needs to be done

[helen]

Holy cow this is fast, and it fixes #60 to boot (had to, because I couldn't use the same path and I figured I might as well fully fix it while I was in here). This has been tested on a super secret live test repo I have.